In recent years, environmental issues such as global warming are gaining attention. Further fuel efficiency for an engine is also demanded in order to address a demand for reduction in CO2 emission into the atmosphere. Further, due to the demand for fuel efficiency, the frictional force between a piston ring and a cylinder liner at the time of sliding is demanded to be reduced. With an increased output of an engine, the quality of a piston ring demanded is increasing, and thus, the sliding characteristics therefor are demanded (for example, wear resistance and scuffing resistance).
In view of such circumstances, a piston ring which has an excellent wear resistance and seize resistance and which maximizes the effect of reducing frictional force, a surface treatment technology and a surface modification technology for such a piston ring are being developed.
Generally, a piston for an internal combustion engine is fitted with three piston rings including a first compression ring as a compression ring, a second compression ring, and an oil ring. For the first compression ring which is exposed to a severe environment, a steel material equivalent to SUS440 (JIS (Japanese Industrial Standards) standard) which is martensite stainless steel or the like is employed. On the other hand, for the second compression ring which has light thermal load or pressure load, a stainless steel material which contains 9-14 mass % of Cr is employed.
In addition, in order to obtain a piston ring which has a good sliding characteristics as mentioned above even when the thermal load or pressure load is increased with higher output, a piston ring base is generally subjected to a nitriding treatment.
However, when at least the outer peripheral sliding surface of the piston ring base is subjected to a nitriding treatment, there have been problems that the dimensional accuracy of the piston ring base decreases and processing after the nitriding treatment becomes very difficult. There has also been drawbacks that, when a nitriding treatment process is performed, many processing processes are needed, which is costly.
The present applicants continues trial and error to resolve such problems concerning such a piston ring for an internal combustion engine and has heretofore been proposed a variety of means for solving the problems.
For example, Patent Document 1 proposes a piston ring made of a carbon steel material composed of C: 0.50 mass %, to 0.75 mass %, Si: 0.15 mass % to 0.35 mass %, Mn: 0.61 mass % to 0.95 mass %, P: 0.03 mass % or less, S: 0.03 mass % or less, the balance Fe and inevitable impurities. In Patent Document 1, the present applicant proposed that at least outside sliding surface of the second compression ring is subjected to a nitriding treatment, or to a hard chromium plating in place of the nitriding treatment.
Patent Document 2 proposes a piston ring which is made of a carbon steel material containing Cr: 9.0 mass % or more, 11.0 mass % or less and C: from 0.45 mass % to 0.55 mass %, and in which the top surface, bottom surface, outer peripheral surface and inner peripheral surface are formed of a nitriding diffusion layer having a hardness of 700 Hv (0.05) or higher in a depth of from 2 μm to 25 μm.
The piston ring proposed in Patent Document 2 is made with the second compression ring in mind. By constituting a piston ring base by a carbon steel material having the above-mentioned composition, a nitriding diffusion layer is formed in a state in which generation of a white layer is inhibited, uniformly without irregularity, thin, and all around the ring. By so doing, a grinding processing after nitriding becomes needless, which reduces cost considerably.